Pst ingestion through inserted emails will be available only on commvault backup media.
SO those ingested emails can be viewed only from commvault consoles.
Not from Outlook Natively(including Addin presence).
As we won't restore the ingested data to Exchange as it will increase the size on exchange.
By using OutlookAddincontentstore they can view the ingested data in seperate folder from Outlook
In Sp18 We also added a feature which will allow the items to restore as Stub to Exchange mailbox, so that you can get a native view from outlook.
Without consuming space on Exchange.